Strengthened cathode material and method of making

ABSTRACT

A CATHODE MATERIAL CONTAINING DISCRETE ISLANDS OF THORIA ENRICHED NICKEL PARTICLES IS DISCLOSED, AS WELL AS A METHOD FOR FABRICATING THE SAME. POWDER METAL TECHNIQUES ARE UTILIZED AND THE MATERIAL COMPRISES A MAJOR AMOUNT OF SUBSTANTIALLY PURE NICKEL AND A MINOR AMOUNT OF A COMPOSITE MATERIAL. THE COMPOSITE MATERIAL COMPRISES COMPOSITE PARTICLES OF CO-TREATED NICKEL AND THORIA. THE THORIA IS HOMOGENEOUSLY DISPERSED WITH RESPECT TO ITS CO-TREATED NICKEL BUT NON-HOMOGENEOUSLY DISPERSED WITH RESPECT TO THE REMAINDER OF THE NICKEL.

y 1973 w. E. BUESCHER ET AL 3,730,706

STRENGTHENED CATHODE MATERIAL AND ME'IHUI) UT" MAKING Filed June 29, 1970 IN Vlz'N 'I() RS WILLIAM E. BucSCHER &

DONALD R. KERSTETTER Ma m 2. 0

ATTORNEY United States Patent 3,730,706 STRENGTHENED CATHODE MATERIAL AND METHOD OF MAKING William E. Buescher and Donald R. Kerstetter, Emporium, Pa., assignors to GTE Sylvania Incorporated,

Seneca Falls, N.Y.

Filed June 29, 1970, Ser. No. 50,843 Int. Cl. B22f 1/00,

U.S. Cl. 75-406 4 Claims ABSTRACT OF THE DISCLOSURE A cathode material containing discrete islands of thoria enriched nickel particles is disclosed, as well as a method for fabricating the same. Powder metal techniques are utilized and the material comprises a major amount of substantially pure nickel and a minor amount of a composite material. The composite material comprises composite particles of co-treated nickel and thoria. The thoria is homogeneously dispersed with respect to its co-treated nickel but non-homogeneously dispersed with respect to the remainder of the nickel.

BACKGROUND OF THE INVENTION This invention relates to strengthened cathode materials and more particularly to powder metal cathodes containing a unique dispersion of a refractory metal oxide. The invention also pertains to methods for making such cathodes.

The cathode has been rightfully called the heart of an electron discharge device. It furnishes, when heated, the electrons which provide the current flow through the device. conventionally, these cathodes, for receiving tubes and cathode ray tubes, are fabricated from a relatively pure nickel material which can be in the form of a tube. Primarily, these tubes are formed from nickel strip material of the order of .002" to .004" thick. The exterior of the cathode is provided with an electron emissive material as is well known in the art. For indirectly heated cathodes, the interior of the tube is provided with a heater, generally comprising a tungsten wire having an insulating coating of aluminum oxide or the like thereabout. When current is passed through the heater the cathode is brought to operating temperature.

The operating temperature of a receiving tube cathode can be of the order of 700 C. to 900 C. and during tube processing, for example on a vacuum pump, the temperature can reach 1100 C. These extreme cathode temperatures cause sufficient elongation or expansion of the cathode so that it can bend or bow to such a degree as to cause shorts to the other electrodes of the tube, for example, the grid, which may be spaced as close as .002" away from the cathode.

The bowing tendency is increased by virtue of the way the cathode is mounted. For proper tube operation the cathode should be rigidly mounted within the tube. This is generally accomplished by mounting the cathode, as well as the other electrodes, between a pair of spaced apart insulating micas. This mica is punched to a correct size to hold these elements in position and the cathode is tightly gripped therein to prevent vibration, which could lead to such problems as microphonism.

Because of these above-cited considerations: i.e., thinness of the cathode material; extremes of operating temperatures; and manner of mounting, cathode bowing has been a persistent problem in tubes having relatively long, thin cathodes, since their introduction. A good deal of time and money has been expended in the search for a solution and, while some progress has been made, the problem still exists.

One of the first major improvements in this area was the introduction of a cathode made by powder metallurgy techniques from a nickel powder material. Prior to this, most cathodes were made from a nickel alloy fabricated by casting processes. One of the most common materials used (and one which is still used in tubes where cathode bowing is not a problem, i.e., tubes having a relatively short cathode) is a #220 alloy available from the International Nickel Company. This material, when made into KY367 cathodes (.050" x .100 x 1.3", thickness .0025") shows an average bend strength of 345 gms. when tested on a GE. bend tester available from the General Electric Company. This apparatus tests the cathode as a beam loaded in the middle and supported at both ends. The cathodes were fired at 1050 C. for 10 minutes in a dissociated ammonia atmosphere, and cooled to room temperature before testing.

By comparison, similar cathode sleeves made by powder material techniques (K5 alloy available from Sylvania Electric Products Inc.) have an average bend strength of 647 gms. under the same test conditions. The results of numerous tests have shown that cathodes of the design of the KY367 having a bend strength of approximately 625 gms. exhibit a markedly reduced tendency to bow; while cathodes having a bend strength of over 800 gms. will not bow under the temperature conditions set forth above. Different cathodes and different applications will, of course, require different bend strengths to pass the minimum tube performance characteristics.

Many attempts have been made in the past to increase the bend strength of cathodes without effecting the elongation characteristics necessary to roll the material to the desired thickness and to allow the jointure or lookseam to be formed. These two results, increased strength and sufficient ductility, as characterized by elongation, are generally at odds with one another and an increase in one has generally meant a sacrifice in the other.

For example, it is known that powder rolled materials may be strengthened by given percentages of at least some refractory oxides, such as thoria. However, producers of these hardened materials have attempted to achieve maximum strength regardless of ductility or workability. Such materials are completely unsatisfactory for lockseam cathode applications simply because they are too hard and springy to form properly on lockseam cathode manufacturing equipment. Furthermore, all known prior techniques of providing thoria as a strengthening agent for nickel cathodes have attempted to achieve a homogeneous dispersion, a condition it is now believed contributed to the loss in elongation characteristics.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, an object of this invention to obviate the disadvantages of the prior art.

It is another object of the invention to enhance nickel cathode materials.

It is another object of the invention to provide a strengthened powder-rolled material while maintaining sufficient elongation to allow ready forming into cathodes.

It is yet another object of the invention to provide a new and novel method of making such material.

It is still another object of the invention to provide a method of providing thoria in a non-homogeneous dispersion in a nickel matrix.

Still another object of the invention is the provision of a non-sintered nickel matrix containing composite particles of a co treated nickel and thoria.

These objects are achieved in one aspect of the invention by the provision of a composition of matter comprising powder-rolled nickel containing isolated areas of co-treated nickel and thoria. The thoria is homogeneously dispersed with respect to its associated co-treated nickel and non-homogeneously dispersed with respect to the remainder of the nickel.

Other objects are achieved by providing a sintered material having a non-homogeneous dispersion of minute thoria particles therein, the thoria being present by between about .05 to .20% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sectional view of a prior art cathode material made by casting processes;

FIG. 2 is a diagrammatic sectional view of a prior art cathode material made by powder rolling techniques;

FIG. 3 is a diagrammatic sectional view of a prior art cathode material containing a homogeneous dispersion of thoria;

FIG. 4 is a diagrammatic sectional view of an embodiment of the invention in the as-rolled condition before sintering; and

FIG. 5 is a similar view of the invention after anneal- DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the abovedescribed drawings.

Referring now to the drawings with greater particularity, there is shown in FIG. 1 a prior art cathode material 6 fabricated from annealed cast nickel material. This material exhibits very large grain growth as shown at 8, which, while giving the material great ductility also contributes to poor bend strength characteristics. Some of the grains 8 will extend across the entire thickness of the material, when formed as cathode strip .002" to .0035" thick, and the strength of the material along these grain boundaries decreases further as the material ages. By way of contrast there is shown in FIG. 2 an enlarged sectional view of a prior art sintered and annealed cathode strip 10 made from powder-rolled nickel particles. Herein, the grains 12 are small and compact and do not extend all the way across the thickness of the material. An example of this type of material is known as K5 alloy and is available from Sylvania Electric Products Inc. This material is composed, by weight, of .001% Al; .10% Co; .15% Mg; .20% Mn; .025% Si; .006% Ti; and the remainder carbonyl nickel. The additives, which perform various functions are not shown in the drawing. Physical- 1y, this material is much stronger than the similar cast material shown in FIG. 1. This material has an average bend strength, when formed into a cathode of the type described above, of about 650 gms. This compares to an average of 345 gms. for a similar melt or cast material. The K5 material has an average tensile strength of about 8000 p.s.i. at 1000 C.; an average yield strength of about 5429 p.s.i. at the same temperature; and an average elongation of about At room temperature the K5 elongation is about 118%. For proper forming into a cathodes a cathode material should have a minimum elongation of at least about 10% at room temperature.

In FIG. 3 there is shown an enlarged sectional view of a prior art sintered cathode strip 14made from powderrolled nickel particles and containing a substantially homogeneous dispersion of about 2% by weight of thoria.

The thoria would also have been added as particles to the original powder blend; however, as can be seen from the illustration (which approximates a photomicrograph having a magnification of 500x) this material discloses no ordinary grain structure even after annealing at high temperatures. (The thoria is not shown in FIG. 3 since a magnification of about 22,000X would be necessary for the thoria to be visible.) In this regard it is quite similar structurally to as rolled regular nickel. The material appears to look like fiber and it is extremely hard. Such a material, wherein the basic matrix is a K5 alloy with the 2% thoria added thereto, has a tensile strength of about 8570 p.s.i.; a yield strength of about 7140 p.s.i.; and an elongation average on a strip rolled to a .010 thickness of about 4% (all measurements being made at 1000 C.). Room temperature elongation measurements were not made with this material because of the difficulty in rolling it below .010. Even at this thickness the material tends to crack during processing because of its lack of ductility.

FIG. 4 represents an enlarged sectional view of one embodiment of the invention. Herein, a non-sintered, powder-rolled strip 20 is shown as comprising nickel particles 22 and a homogeneous dispersion of composite particles 24. The composite particles 24 comprise c0- treated nickel 26 and thoria 28 with the thoria being homogeneously dispersed with respect to its co-treated nickel and being present as about 2-4% by weight of the composite particle. The carbonyl nickel particles utilized in strip 20 have an average particle size of 3.2 microns with a maximum particle size of about 4 microns. This is also true with respect to the carbonyl nickel employed in the previous examples. The composite particles 24 have a particle size in the range of about 1.4-2.6 microns while the thoria 28 has an average particle size of about .015 micron with a maximum size of .05 micron. The thoria particles are shown out of scale in the drawing, which approximates an enlargement of about 500x.

The composite particles 24 are added to the matrix material in an amount to bring the weight percent of thoria in the strip 20 to between .05 and .20.

The composite particles 24 are blended into the nickel particles by conventional powder techniques, for example in a V blender. After blending, the mixture is initially rolled or pressed into strip, bars or rods and then hot or cold rolled to a material which may be substituted for nickel or nickel bearing materials in many applications. For cathode material the mixture is rolled in a strip about .030.040" thick and annealed. During the series of annealing, working and further annealing, the co-treated nickel 26 diffuses into the matrix leaving the strip as shown at 30 in FIG. 5. The grain structure is distinct, as at 31 and the thoria particles 28 are now non-homogeneously dispersed throughout the nickel matrix, in the form of islands. This material, designated KCS with .20% of thoria added, shows many unexpected results. The tensile strength of the material averages 8100 p.s.i. at 1000 C., which is slightly above the K5 material and only a little less than the material which has ten times as much thoria. The yield strength, however, averages 7221 p.s.i., also at 1000 C., which is approximately 33% higher than the K5 and, unexpectedly, is higher also than the prior art material having ten times as much thoria. The elongation average at room temperature for strip .010" thick is about 8.13%, less than that of the K5; however, at .005" thick the room temperature elongation average is 19.03% which is greater than the K5 alone.

The above data for the K5 material and the KCS material of the inventioni(with .20% thoria) are summarized below in Table I. All of the test strips in each of the following tables have been sintered and annealed at least once.

Average Tensile Data for 5 ml. Thick Strip at 20 C.

Tensile Yield strength, strength, Elongap.s.i. p.s.i. tion,

Material X1, 000 X1, 000 percent K5 54. 1 15.7 18. l K5+.05 Th; 53. 4 16. 4 20. 0 K5+.10 Th0: 55.7 18.1 18. 6 K5+.20 Th0: (KOS) 58.8 20. 6 18.5

Reference to Table IE will show that the material of the invention shows significant gains in yield strength over the K5 standard material; and in all instances except with the K5+.05% T1102, there is also a gain in tensile strength. Furthermore, in all of the recited tests at the 5 mil thickness, there is a completely unexpected gain in ductility for the material of the invention. This increase in ductility is not present at the 10 mil. thickness and the reasons therefor are not at all understood at this time.

The exact reason for the results obtained in the increased tensile and yield strength data is also not known; however, it is theorized that the islands of thoria represent points where grain growth is inhibited at the elevated temperatures, and that this prevents the large grain structure which contributes to lowered cathode strength in the processing and operation of receiving tube cathodes. It is also believed that the non-homogeneous dispersion of the thoria allows a certain amount of duetility to be present. This non-homogeneous dispersion allows a certain amount of slippage between the enlarged grains and allows the material to be rolled into a thickness suitable to the formation of cathodes. At the same time the islands of thoria prevent grain growth in the island areas and thus maintains the necessary strength. Cathodes made from this material have a bend strength which can reach 900 gms. and which average 880 gms. after firing at 1100 C. for 10 minutes in dissociated ammonia. This is substantially higher than any other material known which is capable of being fabricated into cathodes by conventional lockseam equipment.

The composite particles, which are the critical elements in this material, can be made by a spray drying technique. Herein a salt solution of nickel and thorium is spray-dried to form homogeneous Ni(NO and Th(NO The nitrates are heated in air to oxidize them to NiO and Th0,, and thereafter they are heated in a reducing atmosphere at a temperature high enough to reduce the NiO to Ni but below the temperature at which the thoria would reduce. Greater details on the formation of the composite particles can be found in Ser. No. 576,857, which is assigned to the assignee of the present invention. It is to be noted that this process is exemplary only and is not critical to the invention described herein. Any other process which would produce the composite particles in the size range specified above would sufiice.

Also, while only one powder material cathode has been described in detail, there are many others which benefit from the strengthening thoria agent. These other cathode materials are primarily nickel; i.e., they are seldom less than about 96% nickel, and they include one or more of various additives added to specifically efiect cathode function. These additives, in various percentages, can be selected from the group consisting of aluminum, cobalt, chromium, copper, iron, manganese, magnesium, silicon, titanium, tungsten, and zirconium.

One such material, for example, is alloy K30 available from Sylvania Electric Products Inc. This material comprises 4% W; .002% A1; .005% C0; .0002% Cu; .003% Fe; .O0l% Mg; .O0l% Mn; .003% Si; .003% Ti; and the remainder nickel.

Thus, it will be seen that there is herein provided a new and novel strengthened cathode material and a method of making the same. It obviates the disadvantages of the prior art. It provides a new and strengthened powder-rolled material which maintains sufiicient elongation to be formed into lockseam cathodes.

A new and novel method of making the material is achieved. Further, there is provided a method of achieving a controllable non-homogeneous dispersion of a strengthening agent in a matrix.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the scope of the invention as defined by the appended claims.

We claim:

1. In a method of making a powder-rolled thermionic cathode material the steps consisting of: adding to a base material comprised of carbonyl nickel particles a quantity of composite particles consisting essentially of cotreated thoria and nickel, said thoria being homogeneously dispersed with respect to said co-treated nickel, mixing said nickel physically and composite particles without melting to form a mixture and rolling and sintering said mixture to form a strip.

2. The invention of claim 1 wherein said carbonyl nickel particles have an average particle size of about 3.2 microns.

3. The invention of claim 2 wherein said composite particles have a particle size in the range of about 1.4 to 2.6 microns.

,4. The invention of claim 3 wherein said composite particles comprise about 24% thoria and said quantity of said composite particles which is added is sufficient to produce a range of about .05 to .20 thoria in said cathode material.

References Cited UNITED STATES PATENTS 3,310,400 3/ 1967 Alexander et a1. -206 3,515,523 6/1970 Galmiche et al. 29-182.5 3,359,100 12/1967 Claus et al. 75-200 3,382,051 5/1968 Barnett 29--182.5 3,137,928 6/1964 Darrah et al. 29-182.5 3,031,740 5/1962 Culbertson et al. 29-182.5

FOREIGN PATENTS 1,115,723 5/ 1968 Great Britain 75200 OTHER REFERENCES Rosenberg, Nickel and Its Alloys, US. Government Printing Ofiice, 1968, pp. 61, 62.

CARL D. QUARFORTH, Primary Examiner B. HUNT, Assistant Examiner US. Cl. X.R. 

